1. Field of the Invention
The present invention relates to an electrode for use in resistance welding of metal plates such as of aluminum alloy, a method of manufacturing the electrode, and a method of resistance-welding workpieces using the electrode.
2. Description of Background Art
As is well known in the art, resistance welding denotes welding processes in which desired regions of metal plates (workpieces) are inserted in superposed relationship between confronting electrodes, and an electric current is supplied to the regions of the metal plates between the electrodes, so that the regions are heated by the electric resistance of the metal plates and welded together. In the case where relatively soft metal plate such as aluminum alloy plates are to be welded by utilizing resistance welding, it is customary to employ electrodes having convex hemispherical tip ends to prevent the metal plates from being deformed or cracked around the welded regions.
In the resistance welding, the electric current flowing from one of the electrodes to metal plates to be welded should preferably be distributed uniformly in a suitable range around the center of the tip end of the electrode. However, the electric current which actually flows tends to concentrate in a limited area (which may not necessarily be the center of the electrode) within the surface of the electrode which is held against the metal plate. In the limited area, fused materials are liable to be attached to the electrodes and the metal plates, or the electrodes and the metal plates tend to stick together. Therefore, if the same electrodes are used, then the number of times that the metal plates can continuously be welded by the electrodes is low, the electrodes have a short service life, and the weld beam on the metal plates does not appear sightly. Another problem is that the electrodes are apt to be worn quickly and damaged in the limited area. When the current concentrates in the limited area of the electrode which is not the center of the electrode, the nugget formed in the welded area of the metal plates is not positioned in a location corresponding to the center of the electrode, and hence no desired weld quality is obtained.
The above problems appear to arise for the following reasons:
Generally, metal plates such as aluminum alloy plates have irregular oxide films formed on their surfaces, and the oxide films serve as an electric resistance to any electric current flowing therethrough. When the electrodes are pressed against the metal plates and supply an electric current to the metal plates, the electric current flowing from one of the electrodes to the metal plates concentrates in a region where the oxide film is relatively thin between the electrode and the metal plate. Thus, the temperature of such a region excessively rises. As a result, the aluminum alloy is diffused from the metal plate into the electrode, alloying the electrode with the aluminum alloy. As the electrodes are used to weld the metal plates at more and more spots, an insulative material such as MgO, Al.sub.2 O.sub.3, or the like is attached to the electrode, or the electrodes and the metal plates stick together. Since the region in which the current concentrates may not necessarily be at the center of the electrode, the nugget produced in the welded region of the metal plates may be positioned not at the center of the electrode.
A microscopic observation of the surfaces of the electrodes and the metal plates generally shows that they have an irregular distribution of minute projections or surface irregularities. When the electrodes are pressed against the metal plates, they are initially held against each other through the minute projections on the surfaces thereof. In the welding process, the welding current concentrates on the small regions where the electrodes and the metal plates are held against each other through the minute projections. Therefore, the temperature of those regions goes excessively high. As a consequence, fused materials of the electrodes and the metal plates are deposited on those small regions. Also since the region in which the current concentrates may not necessarily be at the center of the electrode, the nugget produced by the welded region of the metal plates may be positioned not at the center of the electrode.
To eliminate the above drawbacks, there has been proposed a welding electrode having a surface to be held against a metal plate, which surface is shaped to uniformly destruct an oxide film on the metal plate when it is pressed by the electrode. When the electrode is pressed against the metal plate for welding the latter, the oxide film on the metal plate which is engaged by the electrode is broken off, allowing the electrode to contact a non-oxidized surface of the metal plate in an increased area. Consequently, inasmuch as the current is supplied uniformly from a wide surface area of the electrode and prevented from concentrate in a local region, the service life of the electrode is increased, and a good welding quality is achieved.
Japanese Laid-open patent publication No. 58-159986 discloses one such welding electrode which has a plurality of concentric annular grooves or a spiral groove of triangular or trapezoidal cross section defined in a tip surface thereof which is to be held against a metal plate to be welded. This publication also states the formation of waffle-like grooves in a welding electrode, but does not contain any detailed description of such grooves. The welding electrode with annular grooves or a spiral groove has annular ridges or a spiral ridge having a sharp crest. According to the publication, when the electrode is pressed against a metal plate to weld the latter, since the annular ridges or the spiral ridge destructs any oxide film on the metal plate, the area in which the electrode is held against a non-oxidized surface of the metal plate is increased for effective welding operation.
With the concentric annular grooves or the spiral groove on the electrode, however, the annular ridges or the spiral ridge becomes progressively smaller toward the center of the electrode. Particularly in an initial stage of welding operation, therefore, the welding current is likely to concentrate on a highly localized portion at the center of the electrode. The welding electrode disclosed in the above publication is therefore not effective to solve the above problems.
The concentric annular grooves or the spiral groove is relatively shallow and can easily be filled with fused materials or other deposits. When the annular grooves or the spiral groove is filled with fused materials or other deposits, the welding quality is lowered, and the welded region has irregular mechanical strengths. Any materials attached to or deposited on the tip of the electrode, they cannot easily be removed by a wire brush or the like because they are trapped in the grooves. Since the annular grooves or the spiral groove is formed in the electrode tip by an engine lathe, it is time-consuming to remachine the electrode to reconstruct the annular grooves or the spiral groove.
Japanese laid-open patent publication No. 62-156085 shows a welding electrode having surface irregularities that are formed by shot blasting. According to this publication, the surface irregularities are effective for increasing the service life of the electrode, so that the same electrode may be used to weld workpieces successively a number of times. Because the surface irregularities are formed by shot blasting, however, it is difficult to form convexities of uniform size on the electrode surface. As the surface irregularities are relatively small, when the electrode is used to weld metal plates such as aluminum alloy plates, impurities produced when the metal plates are welded are deposited quickly in the concavities, allowing the electrode to stick easily to the metal plate. Accordingly, the service life of the electrode cannot be increased as expected.